Simulation of emittance dilution in electron storage ring from Compton backscattering

A Monte-Carlo simulation of Compton backscattered λ_L =3.2-μm (E_L=.38494 eV) photons from an IR-FEL on 75-MeV electrons in a storage ring yields an RMS electron energy spread of Δ_E=11.9-keV for a sample of 10⁷ single scattering events. Electrons are sampled from a beam of natural energy spread σ_E=5.6-keV and damped transverse angle spreads σ_x´=.041-mrad and σ_y´=.052-mrad (100%) coupling, scaled from the 200-MeV BNL XLS compact storage ring. The Compton-scattered X-Rays are generated from an integral of the CM Klein-Nishina cross-section transformed to the lab. A tracking calculation has also been performed in 6-dimensional phase space. Initial electron coordinates are selected randomly from a Gaussian distribution of RMS spreads σ_x0=.102-mm, σ_x´0=0.41-mrad, σ_y0=.018-mm, σ _y´0=.052-mrad, σ_φ0a=22-mrad and σ _E0=6 keV. A sample of 10000 electrons were each following for 40000 turns around the ring through an RF cavity of f_rf=211.54-MHz and peak voltage V_m=300-keV. Preliminary results indicate that the resulting energy distribution is quite broad with an RMS width of Δ_E=124-keV. The transverse widths are only slightly increased from their original values, i.e. Δ_x=.106-mm and Δ_x´=.043 mrad. The scaled energy spread of Δ_E~360-keV for ~350,000 turns desired in a 10-msec X-ray angiography exposure is well within the RF bucket used here; even V_m<50-kV is adequate. Further, the electron energy spread adds a negligible RMS X-ray energy spread of Δ_EX=.32-keV. The electron energy damping time of τ_E=379-msec at 75-MeV in an XLS-type ring allows for damping this induced spread and top-off of the ring between heart cycles